In this study, individuals having auditory problems and normal
or near-normal hearing thresholds were divided into groups of
subjects with similar noise exposure, and measured with advanced
hearing tests. For each type of measurement there were individuals
with abnormal results possibly indicating cochlear dysfunction.
For many of the subjects these dysfunctions caused less
improvement than normal when the listening level in the speech
in noise test was increased. Subjects sensitive to loud noise had
significantly better speech recognition in noise at the lower test
level than subjects not sensitive.
There were characteristic results:
Teachers had results suggesting substantial dysfunction in the
auditory system reflected in far worse forward masking and speech
recognition in noise than a group of middle-aged without hearing
problems. These results, suggesting poor temporal processing,
were about equally poor as those of a group exposed to industrial
noise. The latter group was tested for comparison and had about
20 dB worse pure tone thresholds. The matched tinnitus level was
correlated to the forward masking results and those two groups
had the loudest matched tinnitus, possibly caused by dysfunction
in the inner hair cell area.
Musicians showed some deficits normally associated to outer
hair cells and had good results for their age at forward masking,
and so did a group mainly consisting of hospital staff and students.
The musicians had normal speech recognition in noise at both
listening levels.
The study suggests that persons exposed to occupational noise
below or around risk levels may risk hearing dysfunction. Several
of the teachers in this study are examples of that, possibly because
of combinations of unfavourable working environment and
individual susceptibility. Medication or other self-reported nonauditory
factors could not explain the poor results of the teachers.